<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>变量提升问题</title>
</head>
<body>
    
</body>
<script>
    console.log(b) //undefined
    var b = "约吗";
    console.log(a)//Cannot access 'a' before initialization
    let a = "不约";
 </script>
</html>